Mills E. Godwin, one of the last captains of Virginia's Old Guard, sent up the flare yesterday, signaling the state's conservatives that Republican John W. Warner is the U.S. Senate candidate he has chosen to keep the faith.

In a luncheon speech here, Godwin delivered a rolling, frontal assault on Democrat Andrew P. Miller's credibility as a conservative.

"If you don't listen to anything else I say, listen to me now," Godwin implored the already attentive crowd. "I've been in public life for more than three decades and I did not know until this campaign that Mr. Miller was a conservative or that he agreed with our senior United States senator (conservative independent Harry F. Byrd Jr.) on so many points."

Recalling that Virginia had been "under the gun" many times at the hands of the federal government when he was governor and Miller attorney general. Godwin said, "I never knew until he said so this fall that his willingness to stand up and fight the encroachment of federal power was so strong."

There was, of course, no doubt before yesterday that Godwin, 63, supported Warner over Miller. he had appeared for the Republican before. He spoke warmly of him - and coldly of Miller - two weeks ago at a Norfolk breakfast attended by former president Gerald Ford.

But it was not until yesterday, not until the last of the old Byrd Organization governors actually took the lectern in the turn-of-the-century Jefferson Hotel in Richmond that the ritual of Warner's investiture as the conservative candidate was carried out.

The occasion was a luncheon of about 375 Virginians for Warner, an organization whose name is meant to imply that once again Virginia conservatives have banded together to put "principle over party" in their choice of a candidate.

Miller is making a determined effort to woo back conservatives who have drifted away because of disdain for national Democratic leaders and a moderate liberal takeover of the state party organization.

Godwin took on the challenge by painting Miller as a liberal masquerading in conservative cloth.

He warned his audience that liberals "throughout the land" are running as conservatives this year "because this is the year for conservative candidates." Miller, he said, is talking a conservative game because "otherwise he wouldn't have a Chinaman's chance of winning election in Virginia."

Godwin scorned Miller's failure to support Byrd against more liberal opponents and chastised him for endorsing populist Democrat Henry E. Howell last year over Republican Gov. John N. Dalton.

Had Virginia voters listened to Miller in 1977, Godwin said, "John Dalton wouldn't be sitting here today and a lot of us would have wasted a lot of futile effort . . . The Good Lord looks after those who live righteously."

Godwin said Warner "will adorn the halls of Congress" as Virginia conservatives have in the past and will "join hands" with Byrd.

In a brief speech, Warner said to Godwin, "You have spoken of the heart and soul of the Commonwealth. I hear you, sir. I will carry that forward for the next six years."